1. Field of the Invention
The present invention relates to atomic force microscopy systems and, more specifically, to an atomic force microscopy system that compensates for variations in temperature.
2. Description of the Related Art
Atomic force microscopy (AFM) is used to image and otherwise characterize materials at the atomic scale. A typical AFM device includes a cantilever probe or membrane probe that interacts with the material being imaged. Light reflected from the cantilever or membrane is measured either by sensing displacement of the light beam or by interferometry to determine how the probe has interacted with the material.
AFM has been used extensively to probe the nanoscale interactions that take place in wide range of time scales, from microseconds to minutes. Long time scale experiments require stability and control of drift to minimize the effects of changes in ambient conditions. Thermal drift of the cantilever due to ambient temperature changes is a significant source of drift in AFM systems along with mechanical vibrations, material creep, and surface stress changes.
The AFM cantilever is usually a bimorph structure and is sensitive to temperature changes. It can even be used as a thermal detector. In contrast, the deflection of the cantilever due to changes in ambient temperature is detrimental for AFM especially for long time-scale experiments where the rate of drift is comparable with the rate of measured interactions. Thermal drift can be corrected using correlation methods and Kalman filtering for imaging purposes, but a different approach is needed to address this problem for force spectroscopy experiments involving biomolecules or cells. The effect of Thermal drift in these experiments is two-fold: a) the cantilever bends, which can cause a false force reading; and b) the zero-force level shifts. These cannot be tolerated in biomolecular experiments where the samples are delicate and the precise control of both force and tip-to-sample distance is critical. Thus, effective methods for reducing thermal drift in AFM are needed to probe slow biomolecular interactions.
One method reduces thermal drift by simply removing the metal layer over the base of the cantilever. The end of the cantilever, where the deflection is read, still has the metal layer so these cantilevers are still exposed to thermally induced deflection. Instead of modifying the existing cantilevers, one method uses a force sensing structure to effectively reduce the probe dependent thermal. In addition to the efforts for reducing the thermal drift with modified and new probes, researchers have also developed new techniques for existing cantilevers. One method employs a software routine where the cantilever is time-shared between the sample and the substrate for referencing. When the cantilever should be engaged on the sample for the entire experiment, the referencing can be done by reading the deflection of a reference sensor. The reference sensor, which provides distance information from the cantilever substrate-to-sample can simply be another cantilever next to the measurement one, an interferometer, or an electrostatic sensor. Suppression of drift has been demonstrated with these methods which require a feedback controller to keep the force constant. The reference sensor provides information for compensation of drift in distance from cantilever plane to sample substrate. However, this approach may not prevent cantilever bending against a stationary surface while the cantilever is connected to the surface through a biomolecule or a cell.
Thermal drift in AFM systems due to changes in ambient temperature can be a significant source of inaccuracies in AFM measurements. An AFM cantilever is usually a bimorph structure that is sensitive to ambient temperature changes. Such sensitivity can be detrimental in AFM imaging, especially for long term time-scale experiments where the rate of drift may be comparable with the rate of interactions being measured.
In some applications, such as imaging, one can compensate for thermal drift using correlation and filtering. But such compensation schemes may not work well in characterizing biomolecules and cells. This is because these methods do not reduce the additional force generated by the probe resulting from thermal drift. The added force exerted by the probe on the biomolecules and cells resulting from thermal drift can damage or distort such biomolecules and cells.
Therefore, there is a need for a method and device that compensates for thermal drift in AFM, including reducing or eliminating the added amount of force exerted by the probe as a result of thermal drift.